In the course of processing high-purity silicon for applications in semiconductor technology and photovoltaic applications, dealing with silicon melts plays a central role. For example, single crystals for semiconductor technology and photovoltaic applications are pulled from the silicon melts using the Czochralski method. Furthermore, silicon melts can be processed by crystallization using the Bridgman method or by block casting with subsequent controlled crystallization in an economically favourable way to form block material with an advantageous polycrystalline lattice structure, which is subsequently sawed to form silicon wafers for the production of polycrystalline solar cells. Silicon melts also play an important role in various metallurgical processes which are used for cleaning the silicon, for example bubbling various reactive gases through them or slag extraction methods.
In these processes, the use of materials which withstand the attack by the corrosive hot silicon melts and, at the same time, do not release any unacceptable impurities into the ultrapure melt or the melt which is still to be purified is of essential importance.
Usually, for handling ultrapure silicon melts, quartz-glass or fused-silicon instruments are used. Sometimes, however, shaped ceramic articles made of silicon nitride or ceramic containing silicon nitride are also proposed.
On contact between quartz and liquid silicon, however, a chemical reaction takes place with the formation of SiO gas. Owing to this corrosive attack, not only is the material wetted by the melt gradually dissolved and therefore, in the case of prolonged contact times, destroyed, but also in the case of crystallization of the melt or of melt residues in melting pots or moulds, extensive adhesion and baking occurs between the shaped article and the solid silicon. Because of the different coefficients of thermal expansion of quartz and silicon, thermal stresses are created which regularly lead to cracks, flaws and flaking during the cooling of the crystallized silicon. In the case of silicon blocks resulting from controlled crystallization which are intended to be processed further for photovoltaic applications, this phenomenon leads to significant losses of material. In the extreme case, the thermomechanical stresses may be so great that owing to cracks, flaws or flaking, as well as quartz residues baked solid on the block walls, the entire silicon block becomes unusable.
Also in the case of contact between silicon melts and shaped articles made of silicon nitride or ceramic containing silicon nitride, solid baking between crystallized silicon and the ceramic shaped article occurs.
Baking not only leads to considerable losses in the yield of usable silicon, but also prevents reuse of the melting pots. Furthermore, crystallized silicon blocks which have become baked require considerable outlay in order to obtain material which can be used at all. Reliable prevention of such baking is therefore an important contribution for economic production of finished articles made of crystalline silicon.
In order to avoid contact between the silicon melt and the shaped quartz article, Conf. Rec. of 15th Photovolt. Spec. Conference 1981, pp. 576 et seq. and Solar Energy Materials 9, 337-345 (1983) have proposed coating the quartz articles with silicon nitride powder which exhibits a needle- or whisker-like particle morphology.
It has been found, however, that such coating cannot reliably prevent adhesion between quartz or ceramic components and silicon with the described negative consequences primarily when handling relatively large amounts of melt with prolonged contact times between the melt and the shaped article, over the entire contact area between the liquid silicon and the shaped article. Added to this, there are toxicology problems which allow processing of the silicon nitride whiskers safely, as regards occupational hygiene, only with increased outlay on safety techniques, which increases costs.
Furthermore, J. Crystal Growth 94 (1989) p. 62 has proposed to prevent contact between silicon melts or crystallizing silicon and the melting pot by forming an alkaline earth metal halide melt film between the silicon and the melting pot, and in this way to avoid baking. A disadvantage with this procedure is, however, the corrosion, primarily of quartz crucibles, by the alkaline earth metal halide melt in the case of prolonged contact times between the silicon melt and the melting pot, as are technically indispensable in the case of the crystallization of large-format blocks with block courts in excess of 20 kg. Added to this is the inevitable contribution of impurities by alkaline earth metal in the high-purity silicon. Furthermore, it is unavoidably necessary to use high-purity expensive alkaline earth metal halide in order to avoid undesired contamination of the silicon with impurities from the alkaline earth metal halide. Finally, the overall process is very sensitively contingent on the stability of the alkaline earth metal halide melt film between the silicon and the crucible wall; in the event of tearing or insufficient thickness of the melt film, extensive baking to the extent of destroying the crucible is unavoidable.
One possible way of avoiding these disadvantages is provided in U.S. Pat. No. 5,431,869. In that document, a graphite crucible is firstly coated with a silicon nitride powder and then an alkaline earth metal halide melt film is additionally formed between the silicon nitride powder coating and the silicon melt. This procedure is, however, associated with a disadvantageously higher working outlay and likewise contaminates the silicon melt in an undesirable way with alkaline earth metal and, possibly, with other impurities from the alkaline earth metal halide mixture used. Further to this is the fact that an alkaline earth metal halide melt corrodes silicon nitride (see Gmelin Handbook Si Suppl. Vol. B 5 d2, p.214), so that in the case of prolonged contact times between the alkaline earth metal halide melt and the silicon nitride coating--as are unavoidable in the case of the crystallization of large-format silicon blocks--the protective crucible coating can be broken through. In the event of direct contact between the alkaline earth metal halide melt and the quartz crucible, the risk is run that, besides considerable crucible corrosion, the quartz crucible may devitrify even before the silicon crystallization is completed, and be destroyed in this way. This increases the danger of failure of the crucible with uncontrolled outflow of melt, in the case of which the melting system may become damaged, and the crystallized silicon may become unusable for further processing. Protective films of alkaline earth metal halide melts with quartz crucibles therefore appear unsuitable for the handling of silicon melts.
In the case of using alkaline earth metal halide melt films on graphite crucibles, there is the disadvantage that the formation of the crystallized silicon block is only possible above the melting point of the alkaline earth metal halide used. For the production of large-format silicon blocks, however, this is an elaborate and work-intensive procedure which is liable to problems.
There is therefore a need to provide a coating for components which come into contact with initially liquid and subsequently recrystallizing silicon and which reliably prevents adhesion or baking even during the handling of relatively large amounts of melt and the action of liquid silicon on relatively large component areas over prolonged contact times. The coating should furthermore be straightforward and economical to form and equally readily usable on the materials quartz, ceramic and graphite relevant to silicon metallurgy.